Packet data communication system

Abstract: A packet data transmission system is used to link a number of remote hand-held data-gathering units such as bar code readers to a central computer which maintains a database management system. Data packets are sent from the remote units by an RF link to intermediate base stations, then sent by the base stations to the central computer by a serial link. A frequency-hopping transmission method is used for the RF link. The remote units and base stations are all synchronized to a repeating pseudo-random sequence of frequencies in a band (902-928 MHz) which allows this type of transmission. The remote hand-held units initiate an exchange using RF transmission to and from the base stations, receiving only during a rigid time window following a transmission from the remote unit. The base stations cannot initiate communication to the remote units, but instead send data to the remote units only as part of the exchange. The reply messages from the base stations to the remotes contain synchronizing information to maintain coherence with the pseudo-random sequence.

Description: BACKGROUND OF THE INVENTION This invention relates to data communication systems, and more particularly to an RF packet communication system in which a number of remote units send data to a central computer via intermediate base stations. In US-A-5,029,183, a packet data communication system is disclosed using a message-exchange protocol in which a base station only sends a message to a remote terminal in response to a message from the remote terminal. The remote terminal sends a message to the base station only at a time of its own choosing, so the transceiver of the remote terminal need not be continuously powered. The base station responds to a message from a remote terminal within a rigid time window following the message from the remote terminal. If there is a need to send a message originating at the base station (or at a central computer), then the base station must wait until a message is received from the remote before it can send the waiting message as part of its response in the fixed time window. The system of US-A-5,029,183, in an example embodiment, employs spread spectrum RF transmission. The system of US-A-5,029,183 provides a great improvement in the power drain on a battery in a remote terminal, and thus allows construction of smaller, lighter, and cheaper remote terminals. It is a feature of the present invention to provide an improved, low-cost, low-power, packet data communication network in which a number of remote terminal units are in a form of two-way communication with a central station, preferably a network using an RF link or the like so that the remote units may move about freely in an area to be covered by the network. Another feature is to provide an improved packet transmission network in which remote terminal units may be of low cost, low power and small size, yet provide reliable and fast response, as may be needed in a commercial facility (usually indoor) using bar code scanners or the like for data gathering. Another feature is to provide an improved protocol for use in a packet data transmission network which results in reliable and high-performance operation, low power consumption and low cost implementation. A particular feature is to provide an RF data link for portable terminals usable without site licensing under F.C.C. regulations, so that the expense and delays incident to such licensing are eliminated or minimized. Another feature is to provide a method of reducing the delay between message initiation at a base terminal and message receipt at a remote terminal in a system in which only the remote terminals can originate message transmission. A packet data communication system constructed generally as in US-A-5,029,183 includes a number of remote terminal units for gathering data, and a communications link for sending packetized data to a central station and for receiving an acknowledge signal and data from the central station. A packet-exchange protocol ordinarily used for this communications link provides reduced power dissipation at the remote unit by activating the receive function for only a short time, rather than requiring the remote unit to receive or "listen" at all times. To this end, the standard exchange protocol establishes a rigid time window keyed to a transmission by the remote unit, and the remote unit is responsive to a message from the central station only during this time window. The time window is defined to begin at a fixed time delay after a transmission from the remote unit to the central station; at all other times, the receiver is not powered up, except for periodic "listen" times during which the remote listens for packets containing its ID code as one of the remotes for which messages are waiting. In the protocol of US-A-5,029,183, the central station cannot initiate a packet transmission to a remote unit, but instead must wait until the remote unit has sent a transmitted packet, then the central station can reply in the rigid time window, attaching to this base-to-remote packet or acknowledge signal the data it wishes to send to this remote unit. The remote units are low-cost, hand-held units, and so will be of lesser computational capacity than the central station, and power dissipation must be minimized. Accordingly, use of this protocol permits the receive function, and the computation function needed to decode received data, to be scheduled or managed by the remote unit rather than being slaved to the central unit. The central station includes a number of base stations located in different rooms or areas, with all of the base stations connected to a central computer, either by a wire connection or by a similar RF link. At any given time, a remote unit is assigned to only one of these base stations, and as the remote unit moves about it is reassigned to another base station. A feature of the protocol is to include an ID number for the remote unit in the transmitted packet, and to include this same ID number in the reply packet, so acknowledgement by an assigned base station is confirmed. But there need be no address or ID of the base station included in the protocol for communicating with the remote units, since a remote unit is assigned to only one base station, and the base station merely serves as a conduit for communicating with the central computer in any event. The remote terminal units are preferably hand-held bar code readers, and these units are coupled to the central station by an RF link so that the user is free to move about the area of the network. Usually the data packet being sent from the remote unit is the result of scanning a bar code symbol. The reply from the central station in this case would be a validation of the bar code information, or instructions to the user about what action to take regarding the package scanned by the hand-held unit. In the system of US-A-5,029,183, the RF link employs a direct sequence spread spectrum modulation technique to send data packets from the remote terminals to the base stations and return. Spread spectrum methods utilize a transmitted bandwidth much wider than required for the data by adding some coded function to the data, then the received signal is decoded and remapped into the original information bandwidth. A particular advantage of this type of RF data link is that a band may be used which does not require site licensing by the F.C.C., yet it provides reliable, low cost communication from a light-weight, hand-held, battery-operated unit. A "frequency-hopping" RF transmission method is employed which is in accordance with FCC regulations for equipment operating in the 902-928 MHz band. Instead of the direct-sequence spread-spectrum method of US-A-5,029,183, the frequency used by the base stations and remote terminals is periodically switched between fifty different preselected values in this frequency band, in a pseudo-random sequence. All base stations and remotes are of course synchronized with this switching sequence. The sequence of fifty frequencies repeats after switching through the fifty. An interval during which the system dwells at one of the fifty frequencies is long enough to handle a large number of the packet exchanges, as before. BRIEF DESCRIPTION OF THE DRAWINGS The features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, may best be understood by reference to the detailed description of a specific embodiment which follows, when read in conjunction with the accompanying drawings, wherein: Figure 1 is an electrical diagram in block form of a packet data communication system according to one embodiment of the invention; Figure 2 is timing diagram showing events (RF transmission) vs. time for a data transmission sequence in the system of Figure 1; Figure 3 is a more detailed electrical schematic diagram in block form of the host computer and one of the base stations in the system of Figure 1; Figure 4 is a more detailed electrical schematic diagram in block form of one of the remote terminals in the system of Figure 1; Figure 5 is a sectional view of a hand-held bar code scanner unit which may be used as the remote terminal according to one embodiment of the invention; Figure 6 is a view of a part of a bar code symbol to be read by the remote unit of Figures 4 and 5, and a timing diagram of the electrical signal produced thereby; Figure 7 is an expanded view of part of the timing diagram of Figure 2; and DETAILED DESCRIPTION OF A SPECIFIC EMBODIMENT Referring to Figure 1, a data communications network according to one embodiment of the invention is illustrated. A host processor 10 is connected by a communications link 11 to a number of base stations 12 and 13; other base stations 14 can be coupled to the host through the base stations 12 or 13 by an RF link. Each one of the base stations 12, 13 or 14 is coupled by an RF link to a number of remote units 15. In one embodiment, the remote units 15 are laser-scan bar-code readers of the hand-held, battery-operated type as disclosed or referenced in Pat. 5,029,183, for example. Various other types of remote terminals may be advantageously employed in a system having features of the invention; these remote terminals ordinarily would include data entry facilities such as a keyboard or the like, as well as a display (or printer) for indicating to a user information detected, transmitted and/or received by this terminal 15. In this embodiment used as an illustrative example, there may be from one up to sixty-four of the base stations 12, 13 and 14 (three being shown in the Figure), and up to several hundred of the remote units 15; of course, the network may be expanded by merely changing the size of address fields and the like in the digital system, as will appear, but a limiting factor is the RF traffic and attendant delays in waiting for a quiet channel. This communications network as seen in Figure 1 would ordinarily be used in a manufacturing facility, office building complex, warehouse, retail establishment, or like commercial facility, or combination of these facilities, where the bar code readers or similar data-gathering terminals 15 would be used for inventory control in stockroom or receiving/shipping facilities, at checkout (point of sale) counters, for reading forms or invoices or the like, for personnel security checking at gates or other checkpoints, at time clocks, for manufacturing or process flow control, and many other such uses. Although hand-held, laser-scan type bar-code readers are mentioned, the data terminals 15 may also be bar-code readers of the wand type, and may be stationary rather than hand-held. The device may be of the optical character recognition (OCR) type, as well. Other types of data gathering devices may use the features of the invention, such as temperature or pressure measuring devices, event counters, voice or sound activated devices, intrusion detectors, etc. According to an important feature of one embodiment of Pat. 5,029,183, an RF packet communications protocol between the remote units 15 and the base stations 12, 13 and 14 includes a transmit/receive exchange, referred to hereinafter simply as an "exchange". This protocol is similar to collision-sense multiple-access (CSMA) in that a unit first listens before transmitting, and does not transmit if the channel is not free. As seen in Figure 2, this exchange always begins with a remote-to-base transmitted packet 17, representing an RF transmission from a remote unit 15 to be received by the base stations within range. The transmitted packet 17 is followed after a fixed time interval by a base-to-remote transmitted packet 18, representing reception by the remote unit 15 of RF information transmitted by the base station servicing this particular remote unit 15. Each of these packets 17 and 18 is of fixed timing; a transceiver in a remote unit 15 begins an exchange at its own initiative by first listening for other traffic for a brief interval t 0 (typically 0.3 msec), and, if the RF channel is quiet, starting a transmission at a time of its own selection (asynchronous to any clock period of the base stations or host computer). This outgoing transmission packet 17 lasts for a time t 1 as seen in the Figure, and in an example embodiment this period is 4.8 milliseconds. Then at a precise time delay t 2 after it started transmission (e.g., 5-msec after the beginning of t 1 ) the transceiver begins listening for the return packet 18 from the base station. The transceiver in the remote unit 15 only responds to receipt of the packet beginning in a very rigid time window t 3 of a few microseconds length, and if the packet 18 has not started during this window then anything to follow is ignored. The packet 18 is called a "base-to-remote" or "acknowledge" signal, and it may also contain data if the base station has any message waiting to be sent to this remote unit (as well as, in at least some packets, notice of ID numbers of any remote terminals who have messages waiting at the base terminal). The acknowledge packet 18 also is a maximum of 4.8 millisecond in length (in this embodiment), regardless of what data is included, if any, so a remote-to-base exchange, including acknowledge, takes about 9.8 msec in the example. In the system of Pat. 5,029,183, the base stations 12, 13 and 14 ordinarily cannot initiate one of the exchanges of Figure 2, or initiate any other such transmission to the remote units 15. Instead, if a base station has a message waiting to be sent to a remote unit, the base station must wait until a packet 17 is received from this remote unit 15 for which this base station has a message waiting, then the data to be sent is included in the data portion of the return packet 18. For this reason, in the embodiment of Pat. 5,029,183, the remote units 15 were programmed to periodically, e.g., about every 500 msec or more, send a packet 17 to the base station with no data except its identifying code (traditionally called a NOP packet), so that the base station could send any data it has waiting in its memory for relay to this remote unit 15. In the "standard" protocol of Pat. 5,029,183, to prevent another remote unit 15 from starting one of the exchanges of Figure 2 in the interval just after the transmit packet 17 but before the receive packet 18 has started, time t 0 , the listening time, is generally chosen to be longer than the time interval between transmit packet 17 and receive packet 18 (0.2 msec in this example). If another remote unit 15 tries to initiate its own exchange it will receive the RF transmission and will back off and try again at least about 10-msec later. In the manner of CSMA protocols, the remote units 15 can be programmed to wait random time delays before retry, to thereby lessen the likelihood of simultaneous retries. Referring to Figure 3, in a typical commercial or retail application of a network of Figure 1, the host processor 10 maintains a database management system (employing suitable database management software similar to that commercially available) to which the remote units 15 make entries or inquiries via the base stations 12, 13 and 14. The host processor 10 has a CPU 20 which may be an Intel microprocessor device, and the CPU accesses a memory 21 via a main bus 22 to execute instructions, with various I/O processors 23 used to access peripherals such as keyboard, video display, etc., as well as disk storage 24 for the database system and other computer functions. A communications adapter 25 couples the CPU 20 via main bus 22 to the link 11. This communications link 11 may be of the serial type, or in a system designed for higher performance the link 11 may use one of the available local area network type of protocols such as Ethernet or token ring, or may merely use serial port connected to a shared serial line 11, on a time-sharing basis (e.g., time slotted). The base stations 12, 13 and 14 each utilize a CPU 30 which accesses a memory 31 via local bus 32, also seen in Figure 3. This data processing unit is coupled to the serial link 11 via a communications adapter 33. An RF transceiver 34 is coupled to the CPU 30 in each base station via the local bus 32 and is connected to an antenna 35 for RF transmission to and reception from the remote units 15 using the protocol of Figure 2. An additional RF transceiver 34a may be used, as well, as an RF link to and from other base stations, if necessary. An example of a microprocessor device is given in Pat. 5,029,183. The memory 31 includes ROM or EPROM for startup code executed by the CPU 30, as well as fast RAM for the program executed during normal operations and for buffering the digital data incoming from or outgoing to the RF transceiver 34. The base stations 12, 13 and 14 are ordinarily located in various rooms or bays of the commercial establishment containing the network of Figure 1, or located in other such places not readily accessible to an operator, so usually a console with keyboard and display is not used; if, however, a base station is configured for desktop or wall mounting in an accessible location there may be I/O devices coupled to the bus 32 to allow local data entry or display. The base stations are usually powered by line current rather than being battery operated, and so there is less concern for power dissipation in these devices compared to that for the remote terminals 15. The RF signal path in this environment is changeable in nature as equipment, fork-lift trucks, furniture, doors, etc., are moved about, or as the user moves from place to place and carries the hand-held remote unit with him, or as the network is expanded or reduced in size; there is a high degree of multipathing in this type of RF link. Thus, the particular one of the base stations communicating at a given time with one of the remote units 15 may change; to this end a "hand-off" protocol may be utilized, as described in Pat. 5,029,183, to change the base station which is designated to handle a remote unit. In this manner, a remote unit 15 has a confirmed virtual RF link with only one base station at a time, although others may be in range. The base station 12, 13 or 14 is merely an intermediary; the remote unit is communicating with the host processor 10, and the function of a base station is merely to relay the data from a remote unit to the host computer, or from the host computer to a remote unit. In a minimum-scale installation, there may be only one base station, in which case the communication link 11 may be a direct connection, or, if the building is such that the base station can be mounted at the same location as the host computer, the link 11 may be replaced by a parallel bus-to-bus interface, in which case the base station and host computer may be considered a single unit. When there are multiple base stations, the communications link 11 uses a protocol of adequate performance to allow each RF transmission (packet 17) from a remote unit 15 to be decoded in the base station and relayed to the host processor 10 via the link 11, then a reply sent back from the host computer 10 via the link 11 to the base station, so the base station can wait for another exchange for relay of the stored message to the remote unit 15 in a packet 18. Referring to Figure 4, each remote unit 15 in the example embodiment is a data terminal (e.g., a hand-held bar code reader) having a CPU 40 executing instructions from a program and data memory 41 which is coupled to the CPU via a local bus 42. A peripheral bar code data acquisition device 43 is coupled to the CPU via the bus 42 and used to detect and/or convert data from the bar code scanning section to be stored in the memory 41 and processed by the CPU 40; other control devices interface with the keyboard and display. An RF transceiver 44 is coupled to and controlled by the CPU via the bus 42, and transmits the coded RF signal through an antenna 45 or detects and converts RF received by the antenna, according to a protocol. In the example of the remote unit 15 being a laser-scan bar-code reader, the device 43 is used to input data from a photodetector device 46 which produces a serial electrical signal fed to a code recognition circuit 47 responding to the characteristic patterns of bar code symbols and providing bar code data to the memory 41 via device 43 when a bar code is scanned. The CPU 40 within the remote unit checks the bar code data for validity and format, by executing code stored in the memory 41, and, when the data packet has been prepared in the memory 41, the CPU initiates an RF transmission by activating the RF transceiver 44 and transferring the encoded packet containing the bar code data to the transceiver via bus 42. Usually the remote unit has a manual data entry device such as a keyboard 48, and a visual display 49 such as an LCD device; the elements of the keyboard and display are scanned by signals generated in the CPU 40, or generated in a keyboard and display I/O controller. An advantage of the protocol of Figure 2, however, is that the CPU 40 can handle all of these tasks, including data input from the bar code scanner, keyboard and display scan, RF control, datastream transfers to and from the RF, and data encoding and decoding, because the RF transmission and reception is under control of the remote unit rather than being scheduled by a higher level device such as the base station or the host computer. That is, an important feature is the ability of the remote unit 15 to schedule events and communicate with the base station at times of its own choosing, as this ability simplifies the tasks of the remote unit. Therefore, the components needed in the remote unit of Figure 4 are kept to a minimum for cost, size, weight and battery life considerations. The CPU 40 in the remote unit 15 of Figure 4 may be an Intel microprocessor device, having an external bus 42, as described in Pat. 5,029,183. Although other data terminal units 15 may be advantageously employed in a system having features of the invention, a hand-held, laser-scan, bar code reader unit as illustrated in Figure 5 is an example of a remote unit particularly suited for use in the system of Figure 1. This hand-held device of Figure 5 is generally of the style disclosed or referenced in Pat. 5,029,183. A outgoing light beam 51 is generated in the reader 15, and directed to impinge upon a bar code symbol in front of the reader unit. The outgoing beam 51 is scanned in a pattern, and the user positions the hand-held unit so this scan pattern traverses the symbol to be read. Reflected light 52 from the symbol is detected by a light-responsive device 46 in the reader unit, producing serial electrical signals to be processed for identifying the bar code. The reader unit 15 is a gun-shaped device having a pistol-grip type of handle 53, and a movable trigger 54 is employed to allow the user to activate the light beam 51 and detector circuitry when pointed at the symbol to be read, thereby saving battery life if the unit is self-powered. A light-weight plastic housing 55 contains the laser light source, the detector 46, the optics and signal processing circuitry, and the CPU 40 and RF transceiver 44 of Figure 2, as well as a battery. A light-transmissive window 56 in the front end of the housing 55 allows the outgoing light beam 51 to exit and the incoming reflected light 52 to enter. As seen in Figure 5, a suitable lens 57 is used to collimate and focus the scanned beam into the bar code symbol at the proper depth of field, and used to focus the reflected light 52. A light source 58 is positioned to introduce a light beam into the axis of the lens 57 by a partially-silvered mirror and other lenses or beam-shaping structure as needed, along with an oscillating mirror 59 which is attached to a scanning motor 60 activated when the trigger 54 is pulled. The electronic components of Figure 4 are mounted on one or more small circuit boards 61 within the nousing 55 of Figure 5, an batteries 62 are enclosed to provide a self-contained portable unit. The antenna 45 may be printed on one of the circuit boards 61. Alternatively, as described in Pat. 5,029,183, a wand-type bar code reader may be employed instead of the laser scanner of Figure 5. Referring to Figure 6, a part of a typical bar code symbol 65 is illustrated of the type read by the laser scanner of Figure 5, or by a wand-type reader. In the case of a laser scanner, the laser scan beam 51 produces a scan line 66, and reflected laser light 52 detected by the photodetector 46 and shaped by the circuitry 47 produces a binary electrical signal 67 as also seen in Figure 6. Of interest are the transitions 68 in the signal 67 representing passage of the beam or light between light and dark areas or bars and spaces in the symbol 65 (dark produces a binary "0" and light a binary "1" in the illustration). In a typical operation, a user may be located in a receiving or stock room of a commercial or industrial establishment, where this user would aim the remote unit 15 of Figure 5 at a bar code symbol 65 of Figure 6 on a package of interest and pull the trigger 54. This trigger pull initiates a scan operation wherein the laser diode 58 is activated, the scanning motor 60 is energized, the detector 46 is powered-up, and so bar code data is produced for entry to memory 41 via the data acquisition circuitry. This bar code data is loaded to memory 41 then the data may be processed to check for validity, and if valid then a data packet is defined in the memory 41 according to the protocol, then the RF transceiver 44 is activated by a command sent from the CPU 40, the encoded data packet is loaded to the transceiver 44 from the memory 41 in a sequence of bytes, and an RF transmission according to Figure 2 is initiated, i.e., listen during t 0 then transmit a packet 17 if quiet. The base station 12, 13 or 14 receives the RF transmission packet 17 from the remote unit 15, decodes it on the fly, checks for errors, sends an RF acknowledge signal packet 18 to the remote unit 15 during the precise time window, and reformats the data in memory 31 by instructions executed by the CPU 30 for sending to the host computer 10 via communications link 11. The packet 17 from the remote unit 15, or the acknowledge packet 18 from the base station, may contain instructions about when the remote is to initiate another exchange to get the answer to its inquiry. After receiving the data relayed by the base station, the host computer 10 performs whatever database transaction is needed, then sends a response back to the base station via link 11, which the base station holds in memory 31 for sending to the remote unit 15 when another exchange occurs using the protocol of Figure 2. When the remote unit 15 has received the data sent by the host computer in response to its inquiry (relayed via one of the base stations), some indication may be displayed to the user by the LCD display 49 on the hand-held unit of Figure 5. For example, the data sent from the host computer 10 may tell the user of the remote unit 15 to take some action with respect to the package which had the bar code symbol on it just read by the trigger pull, i.e., place the package in a certain bin, etc. For this type of operation, the response time from trigger pull to the answer appearing on the display 49 should be short enough as to be hardly noticeable, e.g., one second or less. The type of operations as just described place several requirements upon the system. First, the remote units should be relatively light in weight and small in size, and of course should not require a wire connection to the central station. Battery operation is thus dictated, but the batteries should not have to be large or heavy, and frequent recharging is to be avoided. Line-of-sight communications, as by an infrared link, are inconvenient in this environment because of obstructions and restrictions in field of view, so RF is preferred. An RF link often imposes the burden of F.C.C. regulations, both for restrictions on the equipment and components employed and frequency bands used, but also for licensing of individual users or sites. The effects of these requirements are minimized as will be seen. A more detailed view of the contents of transmitted packets 17 or 18 is seen in Figure 7. The two packets 17 and 18 are the same in general format, so only one is shown. A packet 17 begins with a start signal 72 of fixed length which is used to give notice to the receiver that a packet is beginning and also to synch the receiver; in addition the start signal may be coded so that only base stations and remote units for this particular network will respond (there may be overlapping networks owned by different businesses). Next, a 3-byte header 73 is sent, and, as seen in the expansion view, the header contains a 13-bit device-identification field 74 or "handle"; each remote unit 15 also has a serial number of 24-bit length so no two remote units 15 need ever be manufactured having the same serial number, but to save needless transmission of data this field 74 is shortened to a 13-bit handle, allowing 2 13 or 8192 remote units to be on one network with unique handles. This handle is given to the remote unit 15 during an initialization or power-up procedure when the remote unit is connected to a base or host by a direct cable. After the device-ID field 74, the header 73 includes a 5-bit "size" field 75 which says how many bytes of data are to follow, the permitted size being from zero to twenty-two bytes of data. Thus a "byte count" type of protocol is used, as has been used in previous packet-type serial communications protocols. The 5-bit "size" field 75 allows 2 5 or thirty-two codes to be sent, but only twenty-three are needed for conveying the size information, so if no size information need be sent in a given packet then other commands or messages can be sent in this field, such as a NOP for merely signalling presence of a remote unit or for allowing the base station to send back data if any is waiting. Next, the header 73 contains two 3-bit fields 76 and 77 representing the record number and acknowledgement number, respectively; if an amount of data exceeding 22-bytes must be sent (as when downloading code from the host computer 10 to a remote unit 15 for execution by the remote's CPU 40 when changing operating modes, for example), then this data is split into 22-byte packets which are numbered consecutively, counting modulo-8, and each must be acknowledged by number. In the usual bar code reading transaction, the packets 17 and 18 will be of 22-bytes or less, so the function of the record and acknowledge counting will be of less importance. Following the header 73, a data field 78 of 0-to-22 bytes is transmitted, and a CRC field 79 concludes the packet. The CRC field contains a calculated function of all the bits of the header field 73 and data field 78 for CRC checking, if the receiving device (remote unit 15 or base station) receives a packet 17 or 18 but the CRC calculation of what is received does not agree with the CRC field 79 received, the packet is discarded and not acknowledged, so it will be resent by the transmitting device after a timeout period. As illustrated in Figure 7, the portion of a packet 17 or 18 after the start symbol 72 is from seven to twenty-nine bytes long; if data to be sent exceeds 22-bytes, then a code can be included in the field 75 (a value above twenty-three) to indicate more is coming. The remote unit 15 does not have to execute extensive computations while it is either transmitting or receiving packets 17 or 18. Instead, the packet 17 is completely made up in the memory 41 before the transceiver 44 is activated, then during the fixed receive window for the packet 18 the incoming data is merely copied to the memory 41 without interpretation, so all decoding or computation is done after the exchange. This remote unit need not be concerned about receiving any further messages from a base station until the remote unit is ready. The remote unit 15 manages or schedules its own packet communication operations, instead of being a slave of the host or base stations. The base stations 12, 13 and 14, on the other hand, must be ready to receive one of the exchanges of Figure 2 at any time, so the transceiver 34 must be activated. at all times, then when a packet 17 is received it must be immediately decoded, checked, acknowledged by a packet 18, and data sent on to the host computer 10; when a reply message is sent back to this base station from the host computer 10 it must be formatted and stored in memory 31 ready to send back to the remote unit 15 when another exchange of Figure 2 is initiated by that remote unit 15. During this time, packets 17 may be received from other remote units, and these must be acknowledged with a packet 18 at the 5-msec timing of Figure 2. Accordingly, the base stations have a much greater computation burden imposed upon the CPU 30, compared to the CPU 40 in a remote unit, and the RF transceiver 34 must operate continuously and recognize incoming signals at any time rather than being off most of the time. The transceiver 34 cannot merely save the received data in memory then shut itself off (the data being evaluated later by the CPU), as is done in the remote units 15. The Pat. 5,029,183 discloses a direct-sequence spread spectrum spread-spectrum RF transmission as a preferred embodiment for the RF link between remote units and base stations. Some method such as this is used in the present embodiment to modulate the RF signal in order to transmit the information needed. Other modulation methods could be used, such as simple FM, or others of known type. Referring to Figure 8, in an embodiment of the present invention using the protocol of Figure 2, a frequency-hopping RF system is employed. The frequency hopping requirements set by the F.C.C. rules demand that a system of this type operate uniformly over fifty different frequencies and that the pattern of different frequencies be pseudorandom. So, for this embodiment, assume that the 902-928 MHz band is broken up into fifty channels (frequencies) as seen on the vertical axis of Figure 8. The channels are numbered 01-50, and an algorithm selects a sequence of these fifty numbers 01-50 from a seed number in a manner similar to the way a chipping sequence operates in the direct sequence spread spectrum system described above That is, a pseudo-random non-repeating sequence of fifty numbers in the range of zero to fifty is generated, where each number represents a different equally-spaced radio frequency in the 902-928 MHz band. This sequence is stored in memory in each of the remote units 15, the base stations 12-14, and the central computer, after initialization of the system. Beginning at time t-0 in Figure 8, the system operates at each of the fifty frequencies of the sequence for 400-ms in a sequence of fifty equal-length time periods 178. All remote terminals 15, base stations and the central computer 10 are synchronized to switch or hop to a new frequency every 400-ms, according to the numbers of the sequence. After the fiftieth frequency in the sequence, at time t-49, the whole sequence begins again, i.e., repeats, hence a time interval 179 for a full cycle takes 0.4X50 = 20-seconds. During an interval 178 that the system is operating at a given one of the fifty frequencies in the sequence, the RF signal is modulated by conventional means, such as FM, or the two-frequency method of Pat. 5,029,183, to convey the bits of message packets 17 and 18. A large number of the exchanges of Figure 2 can occur during a 400-ms period 178 between frequency hops. The method used to- synchronize the remote terminals 15, base stations, and network controller or host computer 10 in this frequency-hopping embodiment uses the host computer 10 as the master clock. The host computer 10 sends out a "start of cycle" message to all of the base stations every 20-seconds, at the beginning of an interval 179, and the base stations reset their local timers at this time. Then, each packet 18 sent out by the base stations to a remote terminal 15 includes a two-byte field in or after the header which defines the "cycle position" in the sequence; here the first 6-bit field of the two bytes define the number in the fifty-frequency cycle (0-49), i.e., in the interval 179, and the next 10-bits define the position in the 400ms time interval 178 in 0.4ms increments (0-through-999). Every remote terminal 15 uses this information to adjust its local timer. In one embodiment of this frequency-hopping method of Figure 8, the circuitry and the chipping sequence disclosed in Pat. 5,029,183 are used with little modification. The chipping clock rate remains the same at 0.667 MHz, and the sampling rate of 2 MHz remains the same. The significant change is in the interpretation of the stream of chips; in the present invention each data bit can be represented by three identical chips (all ones or all zeros). During software despreading (block 145 of Figure 11 of Pat. 5,029,183) in the remote terminal 15, majority logic will be used on the nine samples A, B, C, in the three chips to determine if the bit is a one or a zero. Transmission is similarly changed. The packet header 73 of Figure 7 may be increased in size to include the two-byte "cycle position" field. A longer packet length may also be used. The packet length previously discussed contains 31X8X11=2728 chips plus the header symbol 73; at 222KBps, this represents 113-bytes, or at 333KBps 170-bytes. Each remote terminal can determine the accuracy of its position in each 400ms interval 178 by noting the time since it received an update (a "cycle position" message). Assuming a local clock precision of +/- 40ppm (parts per million), the clock loses +/-1ms or accuracy every 25sec. Therefore, the protocol may be refined by requiring each remote terminal 15 to block-out a time segment at the end of each 400ms interval 179 during which no transmissions should be scheduled due to clock shift; e.g., subtract 2ms for each 25-seconds since the last update. In addition to the block-out for clock drift, the remote terminal IS should not transmit at the end of the 400ms interval 179 if insufficient time is left for a response from the base station (5ms plus the clock drift inaccuracy). The use of the method of Figure 8 requires a change in the distress call procedure, compared to that described in Pat. 5,029,183. A remote terminal 15 knows to what degree it is synchronized to the system's master clock, and can tailor its distress call procedure to minimize the log-on time. It takes approximately 10,000 seconds (2.8 hours) to be off by one 400ms interval 178 (i.e., +/- one interval). Therefore, an intelligent search among the fifty frequencies is in an order based upon the time duration since the last update that this remote logged. A chit-chat exchange, with no data sent in either direction, takes about 1ms (at 222KBps), and so a search among all fifty frequencies should take significantly less than a 400ms interval 178 since carrier sense will be negative for at least forty-nine frequencies. While the invention has been described with reference to a specific embodiments, the description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention. 1. A method of transmitting data packets from a first station to a second station, comprising the steps of: a) transmitting a data packet from said first station to said second station at a selected frequency which is one of a predetermined sequence of frequencies in a band, said selected frequency being changed at regular intervals within a repeat period, said data packet including an indication of a position of said selected frequency in said predetermined sequence and an elapsed time in one of said regular intervals; b) receiving said data packet at said second station at said selected frequency and synchronizing said second station in response to said indication. 2. A method wherein said first station is a base station, and wherein said second station is one of a plurality of remote terminals within range of said base station. 3. A method wherein said one of said regular intervals is a time much longer than the length of said data packet. 4. A method wherein said data packet is sent by said first station in response to a packet sent by said second station. 5. A method wherein said regular intervals are about 400 millisecond. 6. A method wherein said sequence of frequencies is about 50 such frequencies. 7. A method wherein said band is about 902-928 MHz. 8. A data transmission system, comprising: a) a first station having means for transmitting information at a selected frequency which is one of a predetermined sequence of frequencies in a band, said selected frequency being changed at regular intervals, said information including an indication of a position of said selected frequency in said predetermined sequence and an elapsed time in one of said regular intervals; b) a second station having means for receiving said information at said selected frequency and means for synchronizing said second station in response to said indication. 9. A system wherein said first station is a base station, and wherein said second station is one of a plurality of remote terminals within range of said base station. 10. A system wherein said information is a data packet, and said one of said regular intervals is a time much longer than the length of said data packet. 11. A system wherein said data packet is sent by said first station in response to a packet sent by said second station. 12. A system wherein said regular intervals are about 400 millisecond, and said sequence of frequencies includes about 50 such frequencies. 13. A system wherein said band is about 902-928 MHz. 14. A system wherein said data packet sent by said first station is in a time window determined by said packet sent by said second station. 15. A method of transmitting data packets from a remote station to a base station, comprising the steps of: a) transmitting a data packet from said remote station to said base station during a first time period, said data packet being transmitted at a selected radio frequency which is one of a set of different radio frequencies in a fixed band, a different one of said set of radio frequencies in said fixed band being selected at repetitive timing intervals according to a given sequence of selected radio frequencies; b) receiving at said remote station from said base station a reply signal during a second time period after said first time period; said reply signal including an indication of the order of said selected radio frequency in said given sequence and an indication of elapsed time within one of said timing intervals. 16. A method wherein said one of said timing intervals is a time much longer than the length of said data packet. 17. A method wherein said radio frequency is a signal in the band of 902-928 MHz. 18. A method wherein said set includes 50 different frequencies. 19. A method wherein said repetitive time intervals are no more than 400ms.

Claims: A method of transmitting data packets (17, 18) from a first station (12, 13, 14) to a second station (15), comprising the steps of: a) transmitting a data packet (18) from said first station (12, 13, 14) to said second station (15) at a selected frequency which is one of a predetermined sequence of frequencies in a band, said selected frequency being changed at regular intervals within a repeat period, said data packet (18) including an indication of a position of said selected frequency in said predetermined sequence and an elapsed time in one of said regular intervals; b) receiving said data packet (18) at said second stations (15) at said selected frequency and synchronizing said second station (15) in response to said indication. A method according to claim 1 wherein said one of said regular intervals is a time much longer than the length of said data packet (18). A method according to claim 1 wherein said regular intervals are about 400 millisecond. A method according to claim 3 wherein said sequence of frequencies is about 50 such frequencies. A data transmission system, comprising: a) a first station (12, 13, 14) having means (30-35) for transmitting information at a selected frequency which is one of a predetermined sequence of frequencies in a band, said selected frequency being changed at regular intervals, said information including an indication of a position of said selected frequency in said predetermined sequence and an elapsed time in one of said regular intervals; b) a second station (15) having means (44, 45) for receiving said information at said selected frequency and means (40) for synchronizing said second station (15) in response to said indication. A system according to claim 5 wherein said first station (12, 13, 14) is a base station, and wherein said second station (15) is one of a plurality of remote terminals within range of said base station. A system according to any of claims 5-6 wherein said information is a data packet (17, 18), and said one of said regular intervals is a time much longer than the length of said data packet. A system according to claim 7 wherein said data packet (18) is sent by said first station (12, 13, 14) in response to a packet (17) sent by said second station (15). A system according to claim 5 wherein said regular intervals are about 400 millisecond, and said sequence of frequencies includes about 50 such frequencies. A system according to claim 9 wherein said band is about 902-928 MHz. A system according to claim. 7 wherein said data packet (18) sent by said first station (12, 13, 14) is in a time window determined by said packet (17) sent by said second station (15). A method of transmitting data packets (17) from a remote station (15) to a base station (12, 13, 14), comprising the steps of: a) transmitting a data packet (17) from said remote station (15) to said base station (12, 13, 14) during a first time period, said data packet being transmitted at a selected radio frequency which is one of a set of different radio frequencies in a fixed band, a different one of said set of radio frequencies in said fixed band being selected at repetitive timing intervals according to a given sequence of selected radio frequencies; b) receiving at said remote station (15) from said base station (12, 13, 14) a reply signal during a second time period after said first time period; said reply signal including an indication of the order of said selected radio frequency in said given sequence and an indication of elapsed time within one of said timing intervals. A method according to claim 12 wherein said one of said timing intervals is a time much longer than the length of said data packet (17). A method according to claim 13 wherein said radio frequency is a signal in the band of 902-928 MHz. A method according to claim 14 wherein said set includes 50 different frequencies. A method according to claim 15 wherein said repetitive time intervals are no more than 400ms.

Inventors: HEIMAN FREDRIC, TYMES LAROY, HEIMAN, FREDRIC, TYMES, LAROY
Applicants: SYMBOL TECHNOLOGIES INC, SYMBOL TECHNOLOGIES, INC.
Application year: 1994
Main classification: H04L12
IPC codes: H04B1, H04L12, G06K17